Show next scheduled sync time on every page

Exposes the scheduler's next_tick in the topbar via a safe Jinja
helper (falls back to nothing if the scheduler isn't running yet),
and converts the server-rendered UTC timestamp to the visitor's local
time client-side so it reads correctly regardless of timezone.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/web/static/app.js

@@ -0,0 +1,10 @@
+document.addEventListener("DOMContentLoaded", () => {
+    document.querySelectorAll("time[data-utc]").forEach((el) => {
+        const date = new Date(el.dataset.utc);
+        if (Number.isNaN(date.getTime())) {
+            return;
+        }
+        el.textContent = date.toLocaleString(undefined, { dateStyle: "medium", timeStyle: "short" });
+        el.title = `${el.dataset.utc} (UTC)`;
+    });
+});